Explanation:

- A and B together complete 45% of the work in 18 days.

- This means they would complete 100% in $\frac{18}{0.45}=40$ days.

- Therefore, A and B's combined daily work rate is $\frac{1}{40}$ of the work.

- A alone can complete the work in 60 days, so A's daily work rate is $\frac{1}{60}$.

- The daily work rate of B alone, therefore, is $\frac{1}{40}-\frac{1}{60}=\frac{1}{120}$.

- A and B work together for 16 days, completing $16\times \frac{1}{40}=0.4$ or 40% of the work.

- 40% of work is left, since they initially completed 45% and now worked for 16 more days doing 40%.

- B alone will take $\frac{0.60}{\frac{1}{120}}=72$ days to finish 60%.

Correct Answer: Option 3 - 72 days.